A substance only dissolves if the
attraction between the particles of the solute and the solvent is enough
to overcome the attraction of the particles of each substance for
others of the same type. For instance, the attraction between the ions
in table salt and water molecules is enough to both pull individual ions
away from the salt crystal and to separate the water molecules to
accommodate the ions. Benzene, on the other hand, does not attract the
ions of salt strongly enough to remove them from the crystals.
Napthalene does not dissolve in water because napthalene does not exert a
strong enough attraction to water molecules to separate them and make
room for itself.
